It is a warm, close August afternoon and you have a van to unload. Boxes across the driveway, into the unit, roller door down, padlock on.

Nobody will open that door for six months. And whatever the air was carrying that afternoon has just been sealed in with everything you own.

Two separate problems

The air you sealed in. A 30 m³ unit on a muggy day holds a surprising amount of water as vapour — on air at 25°C and 75% humidity, around 520 millilitres. That water does not leave. Whenever the unit cools below its dew point, some of it condenses onto the coldest surfaces available.

The materials themselves. Cardboard, paper, textiles, timber and leather are all hygroscopic — they equalise with the surrounding air. Boxes loaded on a humid day arrive already carrying moisture, and they hold it against whatever is inside them.

Why the unit condenses

A metal-clad storage unit is very good at getting cold.

The roof and walls are thin, uninsulated and face the sky, so they radiate heat away overnight and cool far faster than the dense contents underneath. Once they drop below the dew point of the trapped air, water condenses on the inside of the roof — and then drips.

That is the classic storage failure: a stain down one side of a box that was nowhere near a leak, because the water came from the air rather than from outside.

Seasonal swings make it worse. Warm humid air is sealed in during summer; the unit cools through autumn; the water comes out onto everything.

The mould threshold

Cardboard is an excellent substrate — cellulose, plus whatever glue and print is on it. What it needs is sustained moisture at the surface, around 80% relative humidity for hours at a time, repeated.

A sealed unit that swings through the dew point every night for six months supplies exactly that. And nobody is there to notice.

Loading better

Choose the day where you can. The outdoor dew point is what you will be sealing in. Loading on a dry day genuinely leaves less water in the unit — this is the one decision that costs nothing and is almost never considered.

Load dry things. Anything damp — recently washed textiles, a shed tool that was outside, a fridge that has not fully dried — is a moisture source that stays inside for the duration.

Plastic crates over cardboard for anything vulnerable. Cardboard is cheap and it is also a sponge and a food source.

Off the floor and off the walls. Pallets underneath, a gap behind. Concrete floors wick moisture and walls are the coldest surfaces in the unit.

Desiccant packs in sealed containers with electronics, documents, photographs or leather.

Cover the top, not tightly. A sheet over the stack catches roof drips while still allowing air movement. Sealing the whole stack in plastic traps moisture against the contents, which is worse than the drips.

When climate control is worth it

For wood, upholstery, electronics, paper, photographs, artwork, leather and instruments — usually yes. These are the things that respond to humidity swings by warping, corroding, foxing or delaminating, and none of that is reversible.

For garden tools, plastic storage boxes, ceramics and metal furniture — usually not.

The premium buys a stable indoor environment. Only some of what you own needs one, and knowing which half is the difference between an expensive unit and a wasted one.